Ziegler-Natta processes

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

In order to obtain good mixing of ethene with the catalyst, the original Ziegler-Natta processes used hexane as a solvent. Although the solvent is almost completely recovered, the use of a hazardous material such as hexane detracts from the greenness of the process. Since the catalyst is highly moisture sensitive it needs to be deactivated at the end of the process by addition of water or alcohol, and this produces a small waste... 

Figure 28 Hydrogen transfer reaction in propylene polymerisation by the Ziegler-Natta process.

The importance of Ziegler-Natta processes result from their broad synthesis range possibilities. 

The organometallic chemistry of other members of group IIIA is relatively much less important than that of aluminum. There is an extensive organic chemistry of aluminum, and some of the compounds are commercially important. For example, triethylaluminum is used in the Ziegler-Natta process for polymerization of alkenes (see Chapter 22). 

MPC [Mitsui Petrochemical] A continuous process for polymerizing propylene, based on the Ziegler-Natta process, but using a much more active catalyst so that de-ashing (catalyst removal) is not required. The catalyst contains magnesium in addition to titanium successive versions of it have been known as HY-HS (high yield, high stereospecifity), HY-HS II, and T-catalyst. Developed jointly by Mitsui Petrochemical Industries, Japan, and Montedison SpA, Italy, in 1975, and now licensed in 56 plants worldwide. 

UNIPOL [Union Carbide Polymerization] A process for polymerizing ethylene to polyethylene, and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn. A co-monomer such as 1-butene is normally used. The polyethylene process was developed by F. J. Karol and his colleagues at Union Carbide Corporation the polypropylene process was developed jointly with the Shell Chemical Company. The development of the ethylene process started in the mid 1960s, the propylene process was first commercialized in 1983. It is currently used under license by 75 producers in 26 countries, in a total of 96 reactors with a combined capacity of over 12 million tonnes/y. It is now available through Univation, the joint licensing subsidiary of Union Carbide and Exxon Chemical. A supported metallocene catalyst is used today. 

It might be expected that the bond angle involving the centroids of the cyclopentadienyl rings and the metal center would anticipate the amount of space available to an incoming olefin and, therefore, reasonably expected to correlate with reactivity and/or selectivity of Ziegler-Natta processes. 

The method for the manufacture of polypropylene by the Ziegler-Natta process, which has been in widespread use for several decades, involved some years ago a polymerization in a relatively volatile solvent, for example a light petroleum fraction. That was the drawback of this process, since in the separation and subsequent drying of the polymer formed the solvent could not be completely recovered. Problems are thus experienced in fulfilling environmental protection requirements. An additional obstacle was the large volume of aqueous waste that is generated during workup of the polymer suspension. 

One of the reasons for the importance of TiCl4 is that it reacts with [A1(C2H5)3]2 to generate the effective catalyst that is used in the Ziegler-Natta process for the polymerization of ethylene (see Chapter 21). Halides such as TiCU hydrolyze in water as do most covalent halides ... 

Impact-modified polypropylenes are produced by combining the homopolymer with an ethylene-propylene copolymer rubber. Ziegler-Natta processes yield such products in cascaded reactors. The first reactor in the sequence produces a rigid polymer with a high propylene content and feeds the second reactor, where the ethylene-propylene elastomer is polymerized in intimate mixture with the first material. 

The linear polyethylene produced by the Ziegler-Natta process, called hi li-clensity polyethylene, is a highly ctystalline polymer with 4000 to 7000 ethylene units per chain and molecular weights in the range 100,000 to 200,000 amu. High-density polyethylene has greater strength and heat resistance than the branched product of radical-induced polymerization, called low-density polyethylene, and is used to produce plastic squeeze bottles and molded housewares. 

As it is well known, in the most important Ziegler-Natta processes, the polymer grows as a semi-crystalline powder on the solid catalyst and precipitates in the reaction medium. 

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